In general, the hydroformylation reaction involves the addition of carbon monoxide and hydrogen to an olefin containing a substituent group. In this reaction, two isomers are formed in which one is linear, A, and the other is branched, B. ##STR3##
The history of the utility of hydroformylation reactions has focused principally on the preparation of linear products for subsequent use in making surfactants and detergents; however, the branched product is also very important for use in preparing speciality chemicals or pharmaceutical chemicals.
Selectivity for the branched or the linear isomer is important in producing a single, pure product, and selectivity can be obtained by manipulating the structure of a catalyst for the reaction, which is typically based on rhodium or cobalt.
Through the selection of a single, pure product, a better and more efficient route to the production of .alpha.-aryl aldehydes is obtained, and these aldehydes are precursors to .alpha.-aryl carboxylic acids, such as those forming the base for ibuprofen and naproxen.
Regioselective rhodium catalyzed hydroformylation, as reported by Amer, I., et al, J. Amer. Chem. Soc., 112, 3675 (1990), involves as catalyst a simple zwitterionic .pi.-arene complex of Rh(1,5-cyclooctadiene).sup.+ and BF.sub.4.sup.-. This process is conceptually simple and the selectively, which is typically up to 98% branched, is very good; however, the range of substrates reported therein do not include .beta.-substituents (as in the case of .beta.-methyl styrene).
Drent, E., UK patent appl. GB 2217318, discloses the use of a formula based on aryl phosphite and neutral Rh(I) that provides modest selectivity when the olefin containing substituent is vinyl acetate.
Neibecker, B. et al, J. Org. Chem., 54, 5208 (1989), discloses the use of a system based on a specific phosphine ligand that gives only 80-95% selectivity for the branched isomer.
In the regioselective catalyzed system of Brown, J. M. et al, Tetrahedron, 42, 5105 (1988), reliance is based upon a large, very flexible disaccharide backbone; however, the backbone ligand is not easy to make and it is useful for only a limited range of substrates.
A survey of reactions of simple rhodium carbonyl catalysts that, at certain high pressures and temperatures, is very effective, i.e., up to a 98:2 ratio, is reported in Lazzaroni, R. et al, J. Mol. Cat., 50, 1 (1969).
Other literature and patent references pertaining to selective hydroformylation are as follows: Agency of Industrial Sciences and Technology, JP 48099131; Tanaka, M. et al, Bull. Chem. Soc. Jpn., 47, 1698 (1974); Fujimoto, M. et al., JP 52062233; Pittman, C. U. Jr. et al., J. Org. Chem., et al, 43, 4928 (1978); Lai, V., et al, J. Mol. Catal. 4, 401 (1978); Takeda, et al, JP 54024843; Tinker, Harold B., et al., U.S. Pat. No. 4,268,688; and Hayashi, T. et al, J. Mol. Catal. 13, 323 (1981).